OEIGIlSr OF RIVER TBERAOES. 67 



subteiTanean waters. It then became evident that erosion is often more 

 active from within than from without. Large boihng springs can form only 

 where there is a large surface of porous matter, since the seepage of such 

 matter varies with the surface exposed to the rains. It follows that this 

 kind of erosion ^yas formerly more rapid than at present, since there was 

 then a larger surface exposed. In case of some of the osar-plains the 

 amount of subterranean water must once have been two or more times 

 the present supply. In this connection it should be noted that the rainfall 

 of Maine is from 40 to 56 inches annually. 



ORIGIN OF THE HIGHER RIVER TERRACES OF THE VALLEY DRIFT. 



The following considerations bear on this disputed question : 



1. The facts stated above, and elsewhere, prove that the larger plains 

 of sand and gravel are now being rapidly eroded at considerable distances 

 from streams by rains and subterranean waters. In many cases it can be 

 proved that these agencies are more efficient in eroding high-level terraces 

 than is the meandering stream. 



2. Many of the river terraces which are situated above 230 feet extend 

 continuously down their valleys until they end in terraces in the marine 

 clays. But the latter are plainly due to erosion. 



3. The marine beds have been eroded over areas 1 to 3 and even 6 

 miles broad. A less amount of erosion, though of coarser matter, will 

 account for all the river terraces above the former sea level. 



4. The upper portion of the valley di-ift is so generally coarser than 

 the lower that the conditions for rapid erosion by subterranean waters are 

 afforded by most of the larger valleys of New England. 



5. The formation of terraces and bluffs of erosion not distinguishable 

 in form from the ordinary river teiTace has been observed in recent time. 



Two theories as to the origin of the higher river terraces of valley 

 drift demand examination : One is the erosion theory, according to which 

 the steep bluffs are the result of the partial erosion of a sheet of sediment 

 which once extended across the valley. The other is the theory suggested 

 by Prof J. D. Dana, to account for the terraces of the ujjper Connecticut 

 Valley.^ 



According to the latter theory, the terraces were deposited at the 



'The flood of the Connecticut Valley glacier, Am. Jour. Sci., 3d series, vol. 33, pp. 87, 179, 360, 1882. 



